1. Field of the Invention
This invention relates to the technique of electronic ignition systems of the inductive type for internal combustion engines. In more precise terms, the invention is concerned with an electronic device for controlling the magnetizing current which flows through the primary winding of the ignition coil, a protective circuit being provided in said device for guarding against overvoltages which are encountered under service conditions or which occur accidentally.
2. Description of the Prior Art
A conventional ignition system makes use of an electromechanical contact-breaker, the main disadvantages of which are well known: rapid electrical erosion of contacts under heavy loads, constant closing factor limited by the speed of mechanical opening of contacts, contact-bounce phenomena related to characteristics of mechanical resonance of resilient contact-strips.
In order to overcome the above-mentioned disadvantages of the electromechanical breaker, it has now become a common practice to replace this design by an electronic control device which is not subject to the inherent disadvantages of electromechanical devices. In the case of an electronic device which utilizes a switching transistor, the primary winding of the ignition coil is placed in series with the collector of the transistor and control of the magnetizing current is effected by switching the transistor alternately from the saturating mode in which it is fully turned-on to the nonsaturating mode in which it is fully turned-off. Control of this power switching transistor is usually performed by means of an electronic transducer whilst the means for producing automatic ignition advance can be either of the mechanical type or of the electronic type.
Despite the advantages attached to the use of a switching transistor, it would be unwise to lose sight of the practical limitations resulting from the characteristics of junctions in semiconductors, especially the collector-breakdown d.c. voltage. When the transistor is turned-off, the overvoltage which appears at the terminals of the primary winding undoubtedly depends on the value of the tuning capacitor connected to the terminals of the transistor but is also directly dependent on the load impedance of the secondary winding: spark resistance and spark-plug fouling resistance or else open-circuit resistance resulting from occasional or accidental disconnection of one or a number of spark plugs.
In order to ensure operational safety of the switching transistor, the terminal voltage of said transistor must be limited to a maximum value which is specified for the type of transistor employed. With this objective, it is necessary to adopt means which provide effective protection against overvoltages, taking into account the fast overvoltage rise time (10 to 20 .mu.s) and the peak voltages developed (350 to 500 volts). The protection means now in universal use essentially consist of a Zener diode connected between the collector of the transistor and the control input of said transistor, the knee voltage of said Zener diode being lower than the breakdown voltage of the transistor. The signal appearing at the terminals of the secondary winding of the ignition coil must be capable of attaining a value of the order of 30 kV during normal service. Taking into account the coil transformation ratio of the order of 60 to 100, the value of the signal at the terminals of the primary winding, that is, on the collector of the transistor, is of the order of 300 to 500 V. The knee voltage of the protective Zener diode must be comprised between this value of 300 to 500 V and the breakdown voltage of the transistor. In actual practice, said protective Zener diode must in fact be constituted by two Zener diodes, the knee voltage of which is located in the vicinity of 200 V.
This means of protection by Zener diode suffers from a number of drawbacks: current technologies do not permit integration of Zener diodes with the switching transistor since the temperature coefficient of these diodes varies in inverse ratio to that of the transistor, and the knee voltage of Zener diodes in the category of 200 V and above is widely dispersed. These disadvantages are attended by the following consequences: dispersion of electrical characteristics of control devices and a narrow margin of protection.
The aim of the invention is to overcome the disadvantages of devices protected by Zener diodes in accordance with the prior art. A further aim of the invention is to integrate the protection means with the switching transistor.